Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
The member type was NonnullRefPtr which compares by pointer identity.
This caused false positives in style invalidation when the same values
were recomputed as different objects.
CSS Text 3 gives `text-indent` a couple of optional keywords to control
which lines are affected. This commit parses them, but doesn't yet do
anything with them.
